The SA-MP modification extends the functionality of Grand Theft Auto: San Andreas on PC. It uses other software libraries and has a license agreement that requires a valid GTA:SA license and does not allow commercial use or distribution without permission. It was created by the SA-MP.com team who are not affiliated with Take-Two Interactive or Rockstar.

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The SA-MP modification for Grand Theft Auto: San Andreas (r) is a software
project
aimed at extending the functionality of the Grand Theft Auto: San Andreas (r)
software for Microsoft Windows (r).
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SA-MP CLIENT AND SERVER SOFTWARE LICENSE AGREEMENT AND ATTRIBUTIONS
---- LICENSE TERMS ----
(a) You must have a valid license to use Grand Theft Auto: San Andreas (r) PC
in order for this license to be valid.
(b) The software contained herein is provided on an "as-is" basis without
any form of warranty.
(c) This software may not be exploited for personal, financial or
commercial gain.
(d) The author(s) of this software accept no liability for use/misuse of the
software.
(e) The SA-MP software package may not be distributed, sold, rented or
leased, without written permission of the software author(s).
(f) You may not create or distribute derivative works of the software or files
contained within the package.
(g) You may not use this software for any illegal purposes.
(h) The author(s) of this software retain the right to modify/revoke this
license
at any time under any conditions seen appropriate by the author(s).
(i) Ideas expressed in this software by way of coding or configuration are
property of SA-MP.com.
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(c) 2005-2011 SA-MP.com team.
The SA-MP.com team is not affiliated with Rockstar Games, Rockstar North or
Take-Two Interactive Software Inc.
Grand Theft Auto and Grand Theft Auto: San Andreas are registered trademarks of
Take-Two Interactive Software Inc.
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---- ATTRIBUTIONS ----
- This software makes use of the RakNet networking library (c) 2003 Kevin
Jenkins
- This software makes use of the Pawn language (c) ITB CompuPhase, 1997-2005

2. - This software makes use of the BASS audio library (c) Un4seen Developments Ltd
- This software makes use of the ttmath library (c) 2006-2009, Tomasz Sowa
*** ttmath LIBRARY LICENSE ***
Copyright (c) 2006-2009, Tomasz Sowa
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
Redistributions of source code must retain the above copyright notice,
this list of conditions and the following disclaimer.
Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
Neither the name Tomasz Sowa nor the names of contributors to this
project may be used to endorse or promote products derived
from this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
THE POSSIBILITY OF SUCH DAMAGE.
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